This invention relates to a circuit arrangement for high-frequency operation of a discharge lamp, comprising
input terminals for connection to a supply voltage source, PA1 a load branch provided with terminals for accommodating the discharge lamp and with an electrode heating transformer provided with a primary winding and secondary windings, each secondary winding being shunted by a branch comprising an electrode of the discharge lamp, PA1 at least one switching element for generating a high-frequency current through the load branch from a supply voltage delivered by the supply voltage source, PA1 a control circuit for generating a control signal for rendering the switching element conducting and non-conducting at a high frequency, and PA1 a dimmer circuit coupled to the control circuit for adjusting the frequency of the control signal.
Such a circuit arrangement is known from European Patent 98285. The luminous flux of a discharge lamp operated by means of this known circuit arrangement may be adjusted in that the frequency of the control signal is adjusted. A change in the frequency of the control signal leads to a change in the frequency of the high-frequency current through the load branch, so that the impedance of the load branch and the amplitude of the high-frequency current are also changed. A change in the luminous flux of the discharge lamp may thus be achieved through a change in the frequency of the control signal. In the known circuit arrangement, the electrodes of the discharge lamp are heated during lamp operation both by the high-frequency current flowing through the lamp and by an electrode heating current of the same frequency which flows through the electrodes of the discharge lamp as a result of a potential difference between the ends of the secondary windings of the electrode heating transformer during lamp operation. It is ensured through dimensioning of the known circuit arrangement that the temperature of the lamp electrodes is maintained at a suitable value during a lamp operation in which the discharge lamp achieves the highest adjustable luminous flux as a result of the discharge current and the electrode heating current. Lamp electrode life is comparatively long at this suitable value of the electrode temperature. When the luminous flux of the discharge lamp is reduced by a user by means of the dimmer circuit, however, not only the discharge current through the discharge lamp but also the electrode heating current through the electrodes decreases. The temperature of the electrodes as a result drops further below the suitable value in proportion as the luminous flux of the discharge lamp is reduced further. As a result, lamp electrode life is shortened to a comparatively high degree by dimming of the discharge lamp, while at the same time blackening of the ends of the lamp vessel of the discharge lamp takes place.